1. Field of the Invention
The present invention relates to a method for producing propylene oxide, in more detail, a method for producing propylene oxide from propylene by a one-step process in gas phase with a gold catalyst and molecular oxygen.
2. Description of the Related Art
Propylene oxide is an important chemical feedstock as an universal intermediate for producing chemicals, for example, polyurethane resins, polyester resins, drugs, additives for cosmetics, heat media, solvents, antifreezing fluids (propylene glycol) or the like, which are used when manufacturing various products such as automobiles, electrical appliances etc. As current main industrial production methods of propylene oxide, there are exemplified a chlorohydrin method and an organic peroxide method. In the chlorohydrin method, propylene chlorohydrin is produced from propylene, chlorine and water, and the propylene chlorohydrin obtained is epoxidized with calcium hydroxide. In the organic peroxide method, propylene is epoxidized with an organic peroxide. It is known that t-butanol, cumyl alcohol or styrene is by-produced in the latter organic peroxide method. In the organic peroxide method, alcohol corresponding to the peroxide used is produced as a by-product; accordingly, there was a problem of a market of the by-products or derivatives thereof, that is, a problem of difficulty in utilizing the by-products due to an unbalance between demand and supply thereof. On the other hand, a large quantity of calcium chloride is by-produced and poisonous chlorinated organic compounds are by-produced in the chlorohydrin method. Therefore, a method for producing propylene oxide without accompanying the production of by-products has been studied. A method where no by-product is resultantly produced by returning cumyl alcohol, that is a by-product in the production method using cumene, to cumene again and reusing it (see JP-A No. 2003-81955) and a method where hydrogen peroxide is used as a peroxide so as to form only water as a by-product to eliminate the disposal of a by-product and no other by-product is produced (see JP-A No. 08-127550) have been recently put into operation. However, the method using hydrogen peroxide has a problem of the cost of hydrogen peroxide and a risk problem due to the use of concentrated hydrogen peroxide. Further, in the method reusing cumyl alcohol that is a by-product, there is a problem that hydrogen is necessary when cumyl alcohol is reduced to cumene.
As a method other than the methods mentioned above, a method where propylene and molecular oxygen are directly reacted in a gas phase in the presence of a catalyst is also well known as a laboratory scale method for producing propylene oxide from propylene (see Shigeo T. Oyama, K. Murata, and M. Haruta, Catalysts & Catalysis, 46 (1), pp. 13 to 18 (2004) and “Mechanisms in Homogeneous and Heterogeneous Epoxidation Catalysis”, ed. S. T. Oyama, Elsevir B. V. (2008), pp. 297 to 313). A silver catalyst and a gold catalyst have been extensively studied as a catalyst for the gas phase reaction. Further, it was also reported by the present inventors and others that when propylene is oxidized in the presence of molecular oxygen and hydrogen, propylene oxide can be highly selectively produced when these catalysts are used (see “Mechanisms in Homogeneous and Heterogeneous Epoxidation Catalysis”, ed. S. T. Oyama, Elsevir B. V. (2008), pp. 297 to 313). The method is a method where propylene is directly oxidized with a gold titanium-containing oxide catalyst such as an Au/TiO2 catalyst or an Au/Ti—SiO2 catalyst in the presence of a mixed gas of molecular oxygen and hydrogen. Furthermore, it is also known that Au/titanosilicalite (TS-1) is a gold catalyst excellent in catalyst stability.
In a method where propylene oxide is produced from propylene with a catalyst, oxygen and hydrogen as well as a gold catalyst are necessary. On the other hand, there was recently published a paper concerning the formation of propylene oxide at a propylene-oxygen system, a propylene-oxygen-hydrogen system or a propylene-oxygen-water system with a catalyst obtained by depositing gold clusters of 6 to 10 gold atoms on an amorphous alumina support which is formed on a planar substrate, and the formation of propylene oxide was confirmed in this study (see Sungsik Lee et. al, Angew. Chem., Int. Ed., 48, 1467 to 1471 (2009)). However, the conversion rate of propylene and the formation rates of CO and CO2 are unclear since a catalyst is formed into a thin film. It is, therefore, difficult to discuss the practical significance of this paper. On the other hand, a paper concerning the formation of propylene oxide at a propylene-oxygen-hydrogen system or a propylene-oxygen-water system with a TiO2 catalyst, where gold nano-particles having an average particle diameter of 3.5 nm are deposited, was also published (see Manuel Ojeda and Enrique Iglesia, Chem. Commun, 352 to 354 (2009)). However, the conversion rate to propylene oxide in the paper is very small such as 0.02 to 0.06% and the method is far from a practical level.
Thus, it is necessary to use a stoichiometrical amount of hydrogen in conventional methods for producing propylene oxide. However, hydrogen is expensive and has a risk of explosion. Accordingly, if it is possible to produce propylene oxide by the direct oxidation of propylene at a high conversion rate with a smaller amount of hydrogen than a stoichiometrical amount, propylene oxide can be produced inexpensively and safely from propylene.